1. Field of the Invention
This invention relates to an information reproducing apparatus, and more particularly to an optical pickup for reproducing recorded information from an optical disc on which information is recorded by applying heat thereon.
2. Description of the Prior Art
There are known optical pickups utilizing non-polarizing optical system and completely polarizing optical system. An optical system of optical pickup utilizing non-polarizing optical system is shown in FIG. 1. An optical pickup utilizing non-polarizing optical system generally satisfies the following conditions:
Tp=Ts, PA1 Rp=Rs, and PA1 Tp=100-Rp [%](Ts=100-Rs [%]), PA1 Tp=100 [%], PA1 Rs=100 [%], PA1 TS=0 [%], and PA1 Rp=0 [%]. PA1 ((RP.sub.MIN /LDP.sub.MAX)/OP.sub.LOSS).times.100.ltoreq.Tp [%], PA1 (LP.sub.MON /LDP).times.100.ltoreq.Rp [%], PA1 (PP.sub.MIN /(PP.sub.MAX .times.R.alpha.)).times.100.ltoreq.Rs [%], PA1 Rp=100-Tp [%], and PA1 Ts=100-Rs, PA1 60 [%].ltoreq.Tp.ltoreq.90 [%], PA1 14 [%].ltoreq.Rs [%], PA1 Rp=100-Tp [%], and PA1 Ts=100-Rs,
wherein Tp [%] represents transmittance of polarization prism for p-polarized light, Ts [%] represents transmittance of polarization prism for s-polarized light, Rp [%] represents reflectance of polarization prism for p-polarized light, and Rs [%] represents reflectance of polarization prism for s-polarized light.
An optical system of optical pickup utilizing completely polarizing optical system is shown in FIGS. 2A and 2B. FIG. 2A illustrates a case where optical disc has no double refraction, and FIG. 2C illustrates a case where optical disc has double refraction whose phase contrast is 60 [deg]. An optical pickup utilizing non-polarizing optical system generally satisfies the following conditions:
As illustrated in FIG. 2A, when optical disc has no double refraction, a light incident upon photodetector includes 100% of reflected light of s-polarization and 0% of reflected light of p-polarization. Namely, substantially 100% of light emitted by laser diode is incident upon the photodetector. In contrast, as illustrated in FIG. 2B, when optical disc has double refraction whose phase contrast is 60 [deg], a light incident upon photodetector includes 75% of reflected light of s-polarization (see. FIG. 8), and 25% of light returns to the laser diode. FIG. 2C illustrates quantity of light incident upon photodetector utilizing envelop of RF signal detected.
The optical pickup utilizing non-polarizing optical system has such an advantage that it is not affected by double refraction of material constituting the optical disc. However, the optical disc utilizing non-polarizing optical system has such disadvantages that laser diode becomes noisy because large quantity of light returns thereto, and that light quantity of light incident upon photodetector is small. Particularly, a high-output laser diode is more largely affected by the return light than a low-output laser diode for reproduction only, because the high-output laser diode has small end-face reflectance so as to increase the output power. Further, in a case of optical discs requiring high recording power, such as LDR (Laser Disc Recordable), in which disc is rotated at high speed although reflectance is set relatively low (approximately 40 [%]) for decreasing recording power, it is necessary to ensure high efficiency of optical pickup and to increase incident light quantity of photodetector at the same time. Therefore, it is difficult to apply an optical pickup utilizing non-polarizing optical system to such optical discs.
On the other hand, an optical pickup utilizing completely polarizing optical system has such an advantage that laser diode is less noisy and large quantity of light is incident upon photodetector because all of light from the optical disc is reflected to the photodetector by the polarization prism. However, optical pickup utilizing completely polarizing optical system has such an disadvantage that it is affected by double refraction of material constituting the optical disc. More specifically, light emitted by the laser diode includes 100% of p-polarized light and, if the optical disc has no double refraction, the light is irradiated on the optical disc after passing through the 1/4-wave plate, is reflected by the optical disc and is changed to 100% s-polarized light by again passing through the 1/4-wave plate. As a result, all component of light reflected by the optical disc is reflected by the polarization prism to the photodetector side. In contrast, when the optical disc has double refraction, light reflected by the optical disc becomes elliptically polarized light, and the p-polarization component of which returns to the photodetector after passing through the polarization prism. As a result, noise of the laser diode increases and light quantity incident to the photodetector decreases.
Further, in actuality, EIAJ standard of Laser Disc prescribes that phase contrast of refracted lights by double refraction due to optical disc material should be no more than 40 [deg], and Red Book standardizing format of Compact Disc prescribes that phase contrast of refracted lights by double refraction due to optical disc material should be no more than 46 [deg]. Under these conditions, optical pickup utilizing completely polarizing optical system raises the above mentioned problems. As described above, optical pickups of non-polarizing type and completely polarizing type have both merits and demerits, respectively, and either of them is selectively used to make the best use of its merits.
As an optical pickup for use in recording is required to satisfy both merits of the above-described pickups of two types, however, a conventional optical pickup does not meet this requirement. In this view, it is conceivable to increase efficiency of pickup for ensuring enough laser power for recording, and to increase laser power to compensate for lack of light quantity incident to the photodetector in reproduction. However, a recordable optical disc such as LDR has limit in reproduction laser power and, when the laser power is too high, recorded information may be destroyed. Therefore, such an optical pickup cannot be used for recording.